The processability and mechanical properties of polyethylene polymers have been made blending Cr-catalyzed polyethylene materials with Ziegler-Natta-catalyzed materials, as disclosed in U.S. Pat. No. 4,536,550 and International Patent Application WO 2005/097888. Mechanical blending does not always produce an ideal polymer composition, which has led to methods of producing polymers simultaneously, which may allow for mixing the different polymers at the molecular level. It remains an ongoing challenge to balance the following properties: processability, high die-swell with high-quality surface, dimensional stability, melt-processing at high shear rate values, flow-instabilities, and environmental stress cracking resistance (ESCR).